CMOS image sensors are increasingly being used as low cost imaging devices. A CMOS image sensor circuit includes a focal plane array of pixel cells, each one of the cells includes a photosensor, such as e.g., a photogate, photoconductor, or photosensor having an associated charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel cell may include a transistor for transferring charge from the charge accumulation region to a sensing node, and a transistor for resetting the sensing node to a predetermined charge level prior to charge transference. The pixel cell may also include a source follower transistor for receiving and amplifying charge from the sensing node and an access transistor, for controlling the readout of the cell contents from the source follower transistor.
In a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the sensing node accompanied by charge amplification; (4) resetting the sensing node to a known state; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge from the sensing node.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, which describe the operation of conventional CMOS image sensors and are assigned to Micron Technology, Inc., the contents of which are incorporated herein by reference.
A top-down view of a conventional CMOS pixel cell 10 is shown in FIG. 1. The illustrated CMOS pixel cell 10 is a four transistor (4T) cell. The CMOS pixel cell 10 generally comprises a photosensor, e.g., a photodiode 13, for generating and collecting charge in response to light incident on the pixel cell 10, and a transfer transistor having a gate 7 for transferring photoelectric charges from the photodiode 13 to a sensing node, which is typically a floating diffusion region 3. The floating diffusion region 3 is electrically connected to the gate 27 of an output source follower transistor. The pixel cell 10 also includes a reset transistor having a gate 17 for resetting the floating diffusion region 3 to a predetermined voltage; and a row select transistor having a gate 37 for outputting a signal from the source follower transistor 27 to an output terminal in response to an address signal on gate 37.
FIG. 2 is a cross-sectional view of a portion of the pixel cell 10 of FIG. 1, taken along line 2-2′, showing the photosensor 13 constructed as a photodiode, transfer transistor having a gate 7 and reset transistor having a gate 17. The CMOS pixel cell 10 has a photodiode 13 that may be formed as a pinned photodiode. The illustrated photodiode has a p-n-p construction comprising a p-type surface layer 5 and an n-type photodiode charge collection region 14 within a p-type substrate 2. The photodiode 13 is adjacent to and partially underneath the gate 7 of the transfer transistor. The reset transistor gate 17 is on a side of the transfer transistor gate 7 opposite the photodiode 13. As shown in FIG. 2, the reset transistor includes a source/drain region 32, which is adjacent an isolation region 9. The floating diffusion region 3 is located between the gates 7, 17 of the transfer and reset transistor.
One conventional method for operating the CMOS pixel cell 10 depicted in FIGS. 1 and 2 is illustrated as a timing diagram in FIG. 3. An integration period is initiated for the pixel cell 10 at time T0 after resetting the photodiode 13 and floating diffusion region 3 by turning transfer and reset gate control signals TX and RST to high. Integration thus begins when the transfer transistor and reset gates turn off. During the integration period, electrons are generated by light incident on the photodiode 13 and are stored in the n-type charge collection region 14. These charges are transferred to the floating diffusion region 3 by the transfer transistor when the transfer transistor gate 7 is turned on again, at time, T1. The source follower transistor produces an output signal based on the transferred charges, stored in the floating diffusion region 3. After charge transfer, e.g., at time T2, the row select gate 37 is turned on by applying a row select signal RS. This outputs the signal produced by the source follower transistor to an appropriate column line for readout sampling. It should be noted that FIG. 3 only depicts the timing for the transfer and readout of the photodiode 13 signal. There is typically an additional readout of the floating diffusion region 3 by the row select gate 27 after region 3 is reset (for correlated double sampling or CDS).
One common problem associated with conventional imager pixel cells, such as pixel cell 10, is dark current, that is, current generated due to electron generation/recombination collected in the photodiode 13 in the absence of light. Dark current may be caused by many different factors, including: photodiode junction leakage, leakage along field isolation edges, transistor sub-threshold leakage, drain induced barrier lowering leakage, gate induced drain leakage, trap assisted tunneling, and pixel fabrication defects.
The area directly under the edge of the transfer transistor gatestack 7 is a significant source of dark current. The n-type charge collection region 14 of photodiode 13 is formed close to the surface of the substrate 2 under the transfer gatestack 7 in order to improve transfer efficiency. This causes the photodiode depletion region created during the integration period for the pixel cell 10, and being associated with the n-type accumulation region 14 and the p-type surface region 5, to also be close to the surface of the substrate 2 in this area. This area has a large number of thermally-created electron/hole pairs due to interstitial silicon surface vacancies, especially near the transfer transistor gatestack edge. After reset and during integration, the photodiode 13 is reverse biased, and the electric field created sweeps the thermally created holes into the p-type surface region 5 and the thermally created charge carriers over to the n-type charge collection area 14 of the photodiode 13. These thermally generated charge carriers increase the unwanted dark current for image pixel cell 10 in the area under the transfer gatestack 7.
Another problem associated with conventional transfer gate technology involves fixed pattern noise and lag due to poor charge transfer efficiency. Partially turning on the transfer gate 17 to minimize dark current, however, leads to fixed pattern noise and lag, as the potential barrier of the transfer gate may be too high to fully transfer all of the photo-generated charges.
Accordingly, a pixel cell having efficient charge transfer with minimized dark current, fixed pattern noise, and lag is desired. Also needed is a simple method of fabricating and operating such a pixel cell.